885 research outputs found
The puzzle of metallicity and multiple stellar populations in the Globular Clusters in Fornax
We examine the photometric data for Fornax clusters, focussing our attention
on their horizontal branch color distribution and, when available, on the RR
Lyr variables fraction and period distribution. Based on our understanding of
the HB morphology in terms of varying helium content in the context of multiple
stellar generations, we show that clusters F2, F3 and F5 must contain
substantial fractions of second generation stars (~54-65%). On the basis of a
simple chemical evolution model we show that the helium distribution in these
clusters can be reproduced by models with cluster initial masses ranging from
values equal to ~4 to ~10 times larger than the current masses. Models with a
very short second generation star formation episode can also reproduce the
observed helium distribution but require larger initial masses up to about
twenty times the current mass. While the lower limit of this range of possible
initial GC masses is consistent with those suggested by the observations of the
low metallicity field stars, we also discuss the possibility that the
metallicity scale of field stars (based on CaII triplet spectroscopy) and the
metallicities derived for the clusters in Fornax may not be consistent with
each other. The reproduction of the HB morphology in F2,F3,F5 requires two
interesting hypotheses: 1) the first generation HB stars lie all at "red"
colours. According to this interpretation, the low metallicity stars in the
field of Fornax, populating the HB at colours bluer than the blue side
((V-I)o<=0.3 or (B-V)o<=0.2) of the RR Lyrs, should be second generation stars
born in the clusters;a preliminary analysis of available colour surveys of
Fornax field provides a fraction ~20% of blue HB stars, in the low metallicity
range; 2) the mass loss from individual second generation red giants is a few
percent of a solar mass larger than the mass loss from first generation stars.Comment: 14 pages, 8 figures. Accepted for publication in MNRA
Kinematic segregation of nearby disk stars from the Hipparcos database
To better understand our Galaxy, we investigate the pertinency of describing
the sys tem of nearby disk stars in terms of a two-components Schwarzschild
velocity distributio n.Using the proper motion and parallax information of
Hipparcos database, we determine t he parameters characterizing the local
stellar velocity field of a sample of 22000 disk stars. The sample we use is
essentially the same as the one described by the criteria ad opted to study the
LSR and the stream motion of the nearby stellar populationComment: 19 page
Infrared Photometry of Red Supergiants in Young Clusters in the Magellanic Clouds
We present broad-band infrared photometry for 52 late-type supergiants in the
young Magellanic Clouds clusters NGC 330, NGC 1818, NGC 2004 and NGC 2100.
Standard models are seen to differ in the temperature they predict for the red
supergiant population on the order of 300K. It appears that these differences
most probably due to the calibration of the mixing-length parameter,
, in the outermost layers of the stellar envelope. Due to the
apparent model dependent nature of we do not quantitatively
compare between models. Qualitatively, we find that
decreases with increased stellar mass within standard models. We do not find
evidence for a metallicity dependence of .Comment: 11 pages, 4 figures. AJ accepte
Young Clusters in the Magellanic Clouds II
We present the results of a quantitative study of the degree of extension to
the boundary of the classical convective core within intermediate mass stars.
The basis of our empirical study is the stellar population of four young
populous clusters in the Magellanic Clouds which has been detailed in Keller,
Bessell & Da Costa (2000). The sample affords a meaningful comparison with
theoretical scenarios with varying degrees of convective core overshoot and
binary star fraction. Two critical properties of the population, the
main-sequence luminosity function and the number of evolved stars, form the
basis of our comparison between the observed data set and that simulated from
the stellar evolutionary models. On the basis of this comparison we conclude
that the case of no convective core overshoot is excluded at a 2 sigma level.Comment: 27 pages, 12 figures, AJ accepte
Single and Composite Hot Subdwarf Stars in the Light of 2MASS Photometry
Utilizing the Two Micron All Sky Survey (2MASS) Second Incremental Data
Release Catalog, we have retrieved near-IR magnitudes for several hundred hot
subdwarfs (sdO and sdB stars) drawn from the "Catalogue of Spectroscopically
Identified Hot Subdwarfs" (Kilkenny, Heber, & Drilling 1988, 1992). This sample
size greatly exceeds that of previous studies of hot subdwarfs. Examining 2MASS
photometry alone or in combination with visual photometry (Johnson BV or
Stromgren uvby) available in the literature, we show that it is possible to
identify hot subdwarf stars that exhibit atypically red IR colors that can be
attributed to the presence of an unresolved late type companion. Utilizing this
large sample, we attempt for the first time to define an approximately volume
limited sample of hot subdwarfs. We discuss the considerations, biases, and
difficulties in defining such a sample.
We find that, of the hot subdwarfs in Kilkenny et al., about 40% in a
magnitude limited sample have colors that are consistent with the presence of
an unresolved late type companion. Binary stars are over-represented in a
magnitude limited sample. In an approximately volume limited sample the
fraction of composite-color binaries is about 30%.Comment: to appear in Sept 2003 AJ, 41 pages total, 12 figures, 2 tables are
truncated (full tables to appear in electronic journal or available by
request
The Red Giant Branch in Near-Infrared Colour-Magnitude Diagrams. II: The luminosity of the Bump and the Tip
We present new empirical calibrations of the Red Giant Branch (RGB) Bump and
Tip based on a homogeneous near-Infrared database of 24 Galactic Globular
Clusters. The luminosities of the RGB Bump and Tip in the J, H and K bands and
their dependence on the cluster metallicity have been studied, yielding
empirical relationships. By using recent transformations between the
observational and theoretical planes, we also derived similar calibrations in
terms of bolometric luminosity. Direct comparison between updated theoretical
models and observations show an excellent agreement. The empirical calibration
of the RGB Tip luminosity in the near-Infrared passbands presented here is a
fundamental tool to derive distances to far galaxies beyond the Local Group, in
view of using the new ground-based adaptive optics facilities and, in the next
future, the James Webb Space Telescope.Comment: 7 pages, 7 figures, accepted for publication in MNRA
Hot Horizontal-Branch Stars: The Ubiquitous Nature of the "Jump" in Stromgren u, Low Gravities, and the Role of Radiative Levitation of Metals
A "jump" in the BHB distribution in the V, u-y CMD was recently detected in
the GC M13. It is morphologically best characterized as a discontinuity in u,
u-y, with stars in the range 11,500<Teff(K)<20,000 deviating systematically
from (in the sense of appearing brighter and/or hotter than) canonical ZAHBs.
We present u, y photometry of 14 GCs obtained with 3 different telescopes
(Danish, NOT, HST) and demonstrate that the u-jump is present in every GC whose
HB extends beyond 11,500K, irrespective of [Fe/H], mixing history on the RGB,
and other GC parameters. We suggest that the u-jump is a ubiquitous feature,
intrinsic to all HB stars hotter than 11,500K. We draw a parallel between the
ubiquitous nature of the u-jump and the problem of low measured gravities among
BHB stars. We note that the "logg-jump" occurs over the same temperature range
as the u-jump, and that it occurs in every metal-poor GC for which gravities
have been determined--irrespective of [Fe/H], mixing history on the RGB, or any
other GC parameters. Furthermore, the u-jump and the logg-jump are connected on
a star-by-star basis. The two are likely different manifestations of the same
physical phenomenon. We present a framework which may simultaneously account
for the u-jump and the logg-jump. Reviewing spectroscopic data for several
field BHB stars, as well as two BHB stars in the GC NGC 6752, we find evidence
that radiative levitation of heavy elements takes place at Teff>11,500 K,
dramatically enhancing their abundances in the atmospheres of BHB stars in the
"critical" temperature region. Model atmospheres taking diffusion effects into
account are badly needed, and will likely lead to better overall agreement
between canonical evolutionary theory and observations for BHB stars.Comment: ApJ, Main Journal, accepted. Contains several changes and update
NGC 2419: a large and extreme second generation in a currently undisturbed cluster
We analyse complementary HST and SUBARU data for the globular cluster NGC
2419. We make a detailed analysis of the horizontal branch (HB), that appears
composed by two main groups of stars: the luminous blue HB stars ---that extend
by evolution into the RR Lyrae and red HB region--- and a fainter, extremely
blue population. We examine the possible models for this latter group and
conclude that a plausible explanation is that they correspond to a significant
(~30 %) extreme second generation with a strong helium enhancement (Y~0.4). We
also show that the color dispersion of the red giant branch is consistent with
this hypothesis, while the main sequence data are compatible with it, although
the large observational error blurs the possible underlying splitting. While it
is common to find an even larger (50 -- 80) percentage of second generation in
a globular cluster, the presence of a substantial and extreme fraction of these
stars in NGC 2419 might be surprising, as the cluster is at present well inside
the radius beyond which the galactic tidal field would be dominant. If a
similar situation had been present in the first stages of the cluster life, the
cluster would have retained its initial mass, and the percentage of second
generation stars should have been quite small (up to ~10 %). Such a large
fraction of extreme second generation stars implies that the system must have
been initially much more massive and in different dynamical conditions than
today. We discuss this issue in the light of existing models of the formation
of multiple populations in globular clusters.Comment: 14 pages, 14 figures (5 in low resolution format), 3 tables, accepted
for publication in MNRA
Mass loss along the red giant branch in 46 Globular Clusters and their multiple populations
The location of Galactic Globular Clusters' (GC) stars on the horizontal
branch (HB) should mainly depend on GC metallicity, the "first parameter", but
it is actually the result of complex interactions between the red giant branch
(RGB) mass loss, the coexistence of multiple stellar populations with different
helium content, and the presence of a "second parameter" which produces
dramatic differences in HB morphology of GCs of similar metallicity and ages
(like the pair M3--M13). In this work, we combine the entire dataset from the
Hubble Space Telescope Treasury survey and stellar evolutionary models, to
analyse the HBs of 46 GCs. For the first time in a large sample of GCs, we
generate population synthesis models, where the helium abundances for the first
and the "extreme" second generations are constrained using independent
measurements based on RGB stars. The main results are: 1) the mass loss of
first generation stars is tightly correlated to cluster metallicity. 2) the
location of helium enriched stars on the HB is reproduced only by adopting a
higher RGB mass loss than for the first generation. The difference in mass loss
correlates with helium enhancement and cluster mass. 3) A model of "pre-main
sequence disc early loss", previously developed by the authors, explains such a
mass loss increase and is consistent with the findings of multiple-population
formation models predicting that populations more enhanced in helium tend to
form with higher stellar densities and concentrations. 4) Helium-enhancement
and mass-loss both contribute to the second parameter.Comment: 29 pages, 30 figures, 4 tables. Accepted for publication in MNRA
The early evolution of Globular Clusters: the case of NGC 2808
Enhancement and spread of helium among globular cluster stars have been
recently suggested as a way to explain the horizontal branch blue tails, in
those clusters which show a primordial spread in the abundances of CNO and
other elements involved in advanced CNO burning (D'Antona et al. 2002). In this
paper we examine the implications of the hypothesis that, in many globular
clusters, stars were born in two separate events: an initial burst (first
generation), which gives origin to probably all high and intermediate mass
stars and to a fraction of the cluster stars observed today, and a second,
prolonged star formation phase (second generation) in which stars form directly
from the ejecta of the intermediate mass stars of the first generation. In
particular, we consider in detail the morphology of the horizontal branch in
NGC 2808 and argue that it unveils the early cluster evolution, from the birth
of the first star generation to the end of the second phase of star formation.
This framework provides a feasible interpretation for the still unexplained
dichotomy of NGC 2808 horizontal branch, attributing the lack of stars in the
RR Lyr region to the gap in the helium content between the red clump, whose
stars are considered to belong to the first stellar generation and have
primordial helium, and the blue side of the horizontal branch, whose minimum
helium content reflects the helium abundance in the smallest mass
(~4Msun)contributing to the second stellar generation. This scenario provides
constraints on the required Initial Mass Function, in a way that a great deal
of remnant neutron stars and stellar mass black holes might have been produced.Comment: 23 pages, 7 figures, in press on The Astrophysical Journa
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